The Influence of Transport Phenomena on the Fluid/zed Bed Combustion of a Single Carbon Particle

The burning rate and temperature of the carbon particles are known to affect the efficiency of a fluidized bed combustor, and also the emission levels of undesired noxious components. The main results of an extensive study on the fluidized bed combustion bchaviour of a single carbon particle [I] arc summarized. Calculations have been carried out with a newly developed transient model, the ASPC model, and also with the much simpler progressive conversion model. Besides, many experiments have bccn performed in a lab-scale fluid bed construction to measure the burning rate and temperature of individual carbon particles for various conditions. From the comparison between experimental results and model predictions it has been overall concluded that the ASPC model is especially useful in i) describing the complex bchaviour of progressive carbon conversion for the regime of combustion controlled by carbon reactivity plus intraparticle oxygen diffusion, and ii) estimalJng the conditions for which ~ansition to the regime of external mass and heat transfer control occurs. Accurate prediction of the carbon particle burning rate and temperature is only possible for the latter combustion regime. SCOPE, CONDITIONS Graphite spheres (density: 1780 kg/m 3) arc used as a fuel material. In contrast with coal-derived chars, graphite is a well-defined and uniform material for which combustion experiments yield accurate and reproducible results. The ASPC model is applicable over the entire range of combustion conditions, and basically for all types of carbon. Validation of the model with a limited number of experiments however, is achieved especially by the use of graphite particles. While for certain extreme combustion conditions much simpler models may be used ("shrinking core modcr', "progressive conversion moder'), the ASPC model also allows to estimate possible errors/deviations in~oduccd by the simplifications. This study on the combustion of graphite spheres further shows, in theory and practice, the specific features of pore diffusion limited combustion, thereby facilitating recognition and proper appreciation of similar features in coal-derived char combustion. Extension of the ASPC medcl calculations to the combustion of "rear' char particles is quite well possible, and subject of future work. It should bc noticed however that numerical results arc always limited to the specific char type considered. Wide variations in reactivity and structural properties do not allow a generalized approach in this respect. Conditions of the model calculations are linked-up to those of the laboratory tests (see Table 1). Coefficients for mass and heat exchange between the burning sphere and the fluidized bed are derived from the empirical correlations of Ref. [2,3]. Graphite's reactivity has been determined by thermobalance experiments for pulverized particles. The variables of these kinetic experiments were: i) particle size fraction, ii) temperature T, and iii) gas phase oxygen concentration e. As a result the following expression was derived for the reaction rate on actual mass basis: